Cement restrictor

ABSTRACT

A cement restrictor is provided for creating a fixed obstruction within a bone. An exemplary cement restrictor includes a member or body that is expandable or transitionable from a first diameter to a second diameter.

CROSS REFERENCE RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.08/852,004 filed May 6, 1997, entitled CEMENT RESTRICTOR now U.S. Pat.No. 5,997,580, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/828,035 filed Mar. 27, 1997, entitled BISTABLECEMENT RESTRICTOR now U.S. Pat No. 5,879,403.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH

Not Applicable.

FIELD OF THE INVENTION

The present invention relates to a device used in hip arthroplasty, andmore particularly to a structure for creating a cement restriction orblockage within a bone.

BACKGROUND OF THE INVENTION

Arthroplasty procedures, such as a total hip replacement, can requirethe removal of the femoral head and neck, followed by implantation of anartificial hip stem into a reamed portion of the femoral medullarycanal. Some hip arthroplasty procedures call for the use of bone cementto secure the hip stem within the medullary canal. For procedures thatcall for cement, it is generally undesirable to allow the cements toinfiltrate the medullary canal to an uncontrolled depth and volume.Therefore, a hip arthroplasty procedure can include the step of placingan obstruction within the medullary canal in an attempt to restrict orblock the flow of cement.

Not infrequently, the obstruction is merely a partially hardened orcured ball of cement placed into the canal and held in place by frictionfit with the wall of the canal. This makeshift obstruction is easilydislodged by the distal end of the hip stem if the cement ball is notinserted deep enough into the canal. Additionally, the ball of cement isreadily displaced when pressurized cement is added to the medullarycanal to bind the stem in place. If the cement ball is fractured and/orif it falls beyond a narrow central region of the femur known as theisthmus, the pressurized cement does not properly infiltrate the boneand air pockets or pores are created in the cement. The imperfectionladen hardened cement thus provides a poor interlock with the bone andstem and it is susceptible to cracking. Poor mechanical interlock andcement failure causes the stem to loosen. This undesirable occurrenceoften requires that the joint be replaced in a procedure known as arevision.

Revision surgery and/or procedures requiring a “long” hip stem areespecially problematic with an application that calls for pressurizedcement. Specifically, the distal end of a revision stem ultimatelyextends further into the medullary canal than an original “normal” stembecause additional bone is cut-away during removal of the original stemin preparation to prepare for implantation of the revision stem, or poorquality bone stock forces a larger stem to be used to secure the stemmore distally in the canal to reach better quality bone to achieveimplant stability. Whereas the distal end of the original stem mayextend to a point before or above the isthmus, and thus above the ballof cement, the distal end of the revision stem may extend beyond theisthmus.

Structures other than cement balls are also known for creating ablockage within a medullary canal. For example, FIG. 1 illustrates aknown device 10 including a tapered body 12 having a first end 14, asecond end 16, and fins 18 that extend radially from the body. Each fin18 is resilient and can be flexed toward the first end 14 or the secondend 16 of the body 12 as shown in the illustration by dashed lines.Although it is possible to maintain one or more fins 18 in a flexedcondition by applying pressure to the fin(s) or placing them in aconfined space to elastically deform them, once the pressure is relievedor the device is removed from confinement, the fin(s) will always returnto their original position unless they have been plasticily deformed.Thus, the fins 18 and the device 10 can be described as only having asingle stable state.

In use, a single stable state device 10 can be well suited to the tasksof creating a blockage within a reamed medullary canal 20 above anisthmus region 22 as shown in FIG. 2. It will be noted that the fins 18are deformed different amounts depending on where they are within thetapered medullary canal 20. The body 12 and the fins 18 can have athickness such that even when the fins are fully compressed against thebody, the device 10 is broader than the isthmus 22 to prevent the devicefrom being readily pushed beyond the isthmus. Thus, in a typicalpressurized cement application, the pressurization of the cement doesnot dislodge the device.

By contrast with an above-the-isthmus application, the device 10 istotally unsited for beyond-the-isthmus applications as shown in FIG. 3.Specifically, once some of the fins 18 of the device 10 move beyond theisthmus, there is less and less mechanical interlock with the bone andeven the application of low pressure causes the plug to be dislodged.Were the device 10 to be deliberately passed beyond the isthmus and thenpulled back up into the narrow passage as shown in FIG. 4, the flexedfins 18 would urge the device down and away from the isthmus.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of known techniquesand devices by providing a cement restrictor that is particularly wellsuited for revision arthroplasty. An appropriately dimensioned cementrestrictor can create a fixed obstruction at any selected point within along bone, particularly at points beyond the isthmus.

The cement restrictor includes a single or multiple finned body having afirst stable state and a second stable state. In the first stable state,the cement restrictor is narrower than in the second stable state. Whilethe cement restrictor is readily transitionable from the first stablestate to the second stable state, the transition can be irreversible.

An illustrative embodiment of the cement restrictor includes a bodyhaving a first end and a second end. Bistable fins extend radially fromthe body and are irreversibly movable from a first stable state to asecond stable state. The fins are concave with respect to the first endof the body in the first stable state and convex with respect to thefirst end of the body in the second stable state. The diameter of eachfin is larger in the second stable state than in the first stable state.

Other embodiments of inventive cement restrictors are shown that includeshape memory material that changes shape or dimension(s) in response totemperature and/or stress.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and the attendantadvantages and features thereof will be more readily understood byreference to the following detailed description when it is considered inconjunction with the accompanying drawings, wherein:

FIG. 1 is an elevational view of a prior art cement restrictor;

FIG. 2 is a sectional view of a reamed bone, wherein insertion of theprior art cement restrictor of FIG. 1 is depicted;

FIG. 3 is a sectional view of a reamed bone, wherein the prior artcement restrictor of FIG. 1 has been pushed beyond the isthmus of thebone;

FIG. 4 is a sectional view of a reamed bone, wherein the prior artcement restrictor of FIG. 1 has been pushed completely beyond theisthmus and is being pulled back toward the isthmus;

FIG. 5 is an elevational view of a cement restrictor in accordance withthe present invention in a first stable state;

FIG. 6 is a perspective view of the cement restrictor of FIG. 5;

FIG. 7 is an elevational view of the cement restrictor of FIGS. 5 and 6,showing the cement restrictor in a second stable state;

FIG. 8 is an elevational view of an alternative embodiment of a cementrestrictor in accordance with the invention in a first stable state;

FIG. 9 is an elevational view of yet another embodiment of a cementrestrictor in accordance with the invention in a first stable state;

FIG. 10 shows an exemplary cement restrictor in accordance with theinvention being inserted into a reamed bone portion, wherein the cementrestrictor is in a first stable state;

FIG. 11 depicts the cement restrictor of FIG. 10 in an installedconfiguration beyond the isthmus, wherein the cement restrictor is in asecond stable state;

FIG. 12 depicts another embodiment of a cement restrictor in a firststable state;

FIG. 13 illustrates the cement restrictor of FIG. 12 in a second stablestate;

FIG. 14 depicts another embodiment of a cement restrictor in a firststable state;

FIG. 15 illustrates the cement restrictor of FIG. 14 in a second stablestate;

FIG. 16 depicts another embodiment of a cement restrictor in a firststable state;

FIG. 17 illustrates the cement restrictor of FIG. 16 in a second stablestate;

FIG. 18 depicts yet another embodiment of a cement restrictor in a firststable state;

FIG. 19 shows the cement restrictor of FIG. 18 within a bone as therestrictor transitions to a second stable state;

FIG. 20 depicts a cement restrictor partially cut away to reveal aballoon catheter;

FIG. 21 depicts the balloon of FIG. 20 being inflated to transition thecement restrictor from a first stable state to a second stable state;

FIG. 22 depicts a cement restrictor and a tool;

FIG. 23 depicts the tool of FIG. 22 being used to transition the cementrestrictor from a first stable state to a second stable state;

FIG. 24 illustrates a sheath used to maintain a cement restrictor in areduced diameter state;

FIG. 25 illustrates the cement restrictor of FIG. 24 in an increaseddiameter state;

FIG. 26 shows an internal structure of an embodiment of a cementrestrictor in a first stable state;

FIG. 27 shows a balloon catheter in association with the cementrestrictor of FIG. 26;

FIG. 28 depicts the balloon of FIG. 27 being inflated to transition thecement restrictor from a first stable state to a second stable state;

FIG. 29 depicts the cement restrictor in a stable, expanded state;

FIG. 30 illustrates another embodiment of a cement restrictor in a firststable state;

FIG. 31 illustrates the cement restrictor of FIG. 30 in a secondexpanded, configuration;

FIG. 32 illustrates yet another embodiment of a cement restrictor inassociation with a tool;

FIG. 33 illustrates shape memory structures being extended from an endof the cement restrictor of FIG. 32;

FIG. 34 illustrates the shape memory structures expanding the cementrestrictor;

FIG. 35 illustrates the cement restrictor in an expanded state anddisengaged from the tool;

FIG. 36 illustrates a bistable structure in a first state;

FIG. 37 illustrates a bistable structure in a second state;

FIG. 38 depicts several bistable structures joined together in a reduceddiameter configuration;

FIG. 39 depicts a bistable structure in a reduced diameterconfiguration;

FIG. 40 illustrates the device of FIG. 39 in an increased diameterconfiguration;

FIG. 41 illustrates still another embodiment of a cement restrictor in areduced diameter configuration;

FIG. 42 illustrates the embodiment of FIG. 41 in a first stage of anincreased diameter configuration; and

FIG. 43 illustrates the embodiment of FIG. 41 in a second stage of anincreased diameter configuration.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 5 and 6 are side and perspective views, respectively, of a cementrestrictor 24 in accordance with the invention that includes a body 26from which one or more fins 28 extend radially in a first stable state.As used herein, “stable state” means a condition in which a structure(s)(e.g., the fins) retains a predetermined shape, configuration, ororientation with respect to another element(s) (e.g., the body); andeven if the structure(s) is deformed within a selected range ofdeformation, the structure(s) will rebound or return to thepredetermined shape or configuration in the absence of additional orexternally applied energy or forces. For example, as described ingreater detail below, it can be possible to deform the fins 28 byapplying pressure to them in a first direction, and upon discontinuanceof the pressure, the fins return to their pre-deformation orientation;whereas applying pressure to the fins in a second direction causes thefins to be deformed such that after the pressure has been discontinued,the fins do not return to their pre-deformation orientation.

Continuing to refer to FIGS. 5 and 6, an elongate body 26 has a firstend 30, a second end 32, and an intermediate portion 34 between thefirst and second ends Although each fin 28 can be identicallydimensioned, the exemplary fins 28 are of different diameters. Forexample, the fin 28 near the first end of the body has the smallestdiameter and the fin nearest the second end has the greatest diameter.Each successive fin 28 from the first end of the body to the second endthereof is broader than the preceding fin. Thus, because the body 26 has a uniform diameter, the cement restrictor 24 has a tapered profile.The specific fin dimensions and the overall profile of the cementrestrictor 24 are determined by the anticipated medullary wall contoursat an intended site of obstruction. For an embodiment of the cementrestrictor having fins 28 of different diameters, but havingsubstantially uniform thickness, the broader fins are more flexible thanthe less broad fins to allow the fins to be deformed enough to fitthrough an opening of a selected size, such a reamed isthmus. However,the spacing of the fins 28 from each other inhibits the fins from beingexcessively deformed.

It should be noted that while axial pressure applied to the body 26 inthe direction of the second end of the body, or axial pressure appliedto the fins in the direction of the first end of the body, or acombination thereof, can cause the fins 28 to be deformed, as shown inFIG. 10, the cement restrictor 24 remains in the first stable state. Bycontrast, axial pressure applied to the body in the direction of thefirst end of the body, or axial pressure applied to the fin in thedirection of the second end of the body, or a combination thereof, cancause the fins to be deformed, as shown in FIG. 7, to transition thecement restrictor from the first stable state to the second stablestate. The first stable state of the cement restrictor is notable forthe fins 28 being angled toward the first end of the body or fins whichare convex with respect to the second end of the body to facilitateinsertion of the cement restrictor into a medullary canal. In its secondstable state, shown in FIG. 7, the cement restrictor 24 is notable forthe angulation of the fins toward the second end of the body or finswhich are concave with respect to the second end of the body to inhibitmovement of the cement restrictor with respect to the bone as shown inFIG. 11. The cement restrictor can be configured so as to beirreversible. In other words, it cannot be transitioned from the secondstable state to the first stable state. However, even in the secondstable state the fins can flex, yet return or urge to return a to thepre determined configuration or shape that defines the second stablestate.

The embodiment of the cement restrictor shown in FIGS. 5-7 includeseight fins 28. Although the number of fins many be different for otherembodiments, and can be as few as a single fin, it is desirable to havea large number of fins to maximize the surface for mechanical interlockbetween the fins and the bone, to ensure that the cement restrictor doesnot become displaced during subsequent cement pressurization.

In an exemplary embodiment, the fins 28 are made of a resilient materialsuch as polyethylene and they are joined to or are integral with thebody 26 so as to be bistable as described above. However, the fins 28can also be made of a temperature responsive, stress responsive, orsuper elastic shape memory alloy (SMA) as described below. Thus, thefins 28 can be in the first stable state at a first temperature orstress condition and in the second stable state at a second temperatureor stress condition. In an exemplary embodiment, the cement restrictoris chilled to below (or heated above) body temperature to place it inthe first stable state, at which point the cement restrictor is readilyinsertable into a bone. As the fins warm (or cool) to a temperature inthe normal range of body temperatures, the fins transition to the secondstable state and engage the bone. Additionally, even though the fins areshown as discrete elements, other embodiments include a single, helicalfin.

Continuing to refer to FIG. 5 and 6, the body 26 can include anengagement feature to allow it to be manipulated with surgical tools toposition the cement restrictor and to transition it from the firststable state to the second stable state. As illustrated, the body 26includes a recess or socket 36 into which a tool 38 (shown in FIG. 10)can be inserted to push the cement restrictor 24 through the medullarycanal and with which axial pressure can be applied to the body. Thesocket 36 can include a resilient surface or sleeve to help temporarilyhold the tool 38 in an engaged relationship with the cement restrictor24. In another embodiment, the socket 36 and the tool 38 are threaded.The specific features of the tool and its engagement with the cementrestrictor are not of particular importance with respect to the presentinvention.

Although fins 28 in the second stable state are capable of holding thecement restrictor 24 in place within a bone, other embodiments includefins with roughened peripheral regions, such as the edge of the fin andan adjacent surface portion. In yet other embodiments, such as shown inFIG. 8, barbs 42 can extend from the periphery of one or more fins ofrestrictor 24′. The cement restrictor can be twisted to cause the barbsto dig into the bone. FIG. 9 illustrates yet another embodiment of thecement restrictor 24″ adapted to enhance interlock with a bone surface,wherein cuts 44 extend radially through one or more fins. When thecement restrictor is twisted, the fins separate at the cuts and theedges of the fins dig into the bone.

FIG. 10 illustrates the an exemplary cement restrictor 24 in accordancewith the invention being pushed into a medullary canal with an insertiontool 38. The cement restrictor is in a first stable state anddeformation of fins 28 at the isthmus region should be noted.

FIG. 11 shows the cement restrictor of FIG. 5 in place beyond theisthmus. The insertion tool 38 (or other tool) has applied a tractiveaxial force to the body to cause the fins to transition to a secondstable state, and the cement restrictor is shown in the second stablestate with the tool(s) removed. The fins engage the bone wall withsufficient force to permit pressurized cement to be added to themedullary canal in a manner known to those skilled in the art withoutdislodging the cement restrictor.

In addition to the embodiments illustrated above, other embodiments ofcement restrictors are now described that benefit from the properties ofshape memory materials, as well as traditional materials such as metalwire, to provide structures that can be expanded from a first diameterto a second diameter along a selected axis and/or which can betransitioned from a first stable configuration to a second stable stateconfiguration. The capability of expansion permits a structure to bepassed through an opening or passage, such as an isthmus of a medullarycanal, in a reduced diameter state. Once past the opening or the passage(such as the isthmus), the structure is expanded to a diameter greaterthan that of the opening or passage. Depending upon the application, thestructure can have many uses such as being a plug, a cement restrictor,or an anchor.

For example, FIG. 12 depicts a sheet of shape memory material 40, suchas Nitinol, that has been rolled and/or folded (and trimmed as required)to provide a roughly conical structure 42 having a first diameter. Whenstress on the folded structure is released or reduced, the cementrestrictor 42 expands. Although a shape memory material can be folded ina particular manner as shown with respect to FIG. 12, FIG. 14 depictsanother embodiment wherein a sheet of shape memory material 44 is merelycrumpled to provide a reduced diameter cement restrictor 46 and laterallowed or caused to expand to provide a cement restrictor 48 having anincreased diameter (FIG. 15). Prior to being crumpled, the memorymaterial 44 can be cut or trimmed to cause the material to assume aparticular configuration when it is fully expanded, as shown in FIG. 15.For example, the material 44 can be cut to provide barbs along theperiphery of the cement restrictor.

FIG. 24 illustrates a sheath 50 for constraining a cement restrictor 42in a stressed condition. When the sheath is separated from the cementrestrictor 42, the cement restrictor expands as shown in FIG. 25. In anexemplary procedure, a sheath enclosed cement restrictor 42 is insertedinto the medullary canal of a bone. The cement restrictor is held inposition with a tool (not shown) at a desired obstruction site and thesheath is removed. The cement restrictor expands and forms a blockage inthe bone. Although, the sleeve 50 is a cylindrical body, in otherembodiments it is a band that surrounds only a portion of the cementrestrictor 42.

Whereas FIGS. 12-15, 24 and 25 illustrate embodiments that are placedunder stress, then released and allowed to expand, FIGS. 16-19illustrate embodiments that change shape in response to a temperaturechange. For example, FIG. 16 illustrates a cylindrical structure 52including a memory material. When heated as shown in FIG. 17, portionsof the structure 52 expand in varying degrees to provide a taperedstructure. FIG. 18 depicts a cylindrical structure 54 that transitionsto an expanded stable state upon the application of cold. FIG. 19 showsthe structure 54 within a bone 56, partially covered with crushed ice58.

Yet other embodiments of the cement restrictor are stable in anunconstrained state, and are transitioned to a second stable statethrough the application of stress. For example, FIG. 20 depicts acylindrical cement restrictor 60 partially cut away to reveal a portionof a balloon catheter 62 within a space defined by the cementrestrictor. When a balloon portion 64 of the catheter 62 is inflated, itforces at least a portion of the cement restrictor outward as shown inFIG. 21. When the balloon is deflated, the cement restrictor 60 remainsin the expanded configuration. For a temperature responsive memorymaterial, the balloon could be filled with a heated or cooled fluid.

FIGS. 26-29 illustrate an internal structural element 66 for anembodiment of a cement restrictor such as that shown in FIGS. 20 and 21.The structural element 66 acts as a skeleton around and over which aboicompatible material is formed. Although the structural element 66 canbe a memory material, it can also be a simple metal wire that isdeformable. As illustrated in FIG. 27 the balloon catheter 62 is withina space defined by the cement restrictor. When a balloon portion 64 ofthe catheter 62 is inflated, it forces at least a portion of the cementrestrictor 66 outward as shown in FIG. 28. When the balloon is deflated,the cement restrictor 60 remains in the expanded configuration as shownin FIG. 29.

In still another embodiment, the structure 66 is not initially coveredwith a biocompatible material and it alone is placed into the bone andexpanded as shown in FIG. 28. While the balloon is inflated, bone cementis poured onto the structure 66 and allowed to harden. The balloon issubsequently deflated and a cement restrictor, including bone cementwith a wire reinforcement, remains in place within the bone. A simpleplug or cover (not shown) can be placed in or over any remaining openingdefined by the cement restrictor in this and the other disclosedembodiments. A shield (not shown) can be interposed between the coil andthe balloon to prevent cement from sticking to the balloon.Alternatively, a non-stick coating can be applied to the balloon.

Another way to apply stress to a stress responsive or merely deformablecement restrictor is shown in FIG. 22 which depicts a cement restrictor68 engaged with a tool 70. The tool can be pressed or screwed throughthe cement restrictor to cause it to expand as shown in FIG. 23.

FIG. 30 illustrates yet another embodiment of a cement restrictor thatincludes one or more wires 72 embedded in a biocompatible material 74such as polyethylene. The wire(s) 72 which can include memory materialor ordinary copper, steel or the like, can be oriented longitudinally asshown. Following a temperature or stress change (removal or application)at least a portion of the cement restrictor expands.

FIGS. 32-35 illustrate yet another embodiment of a cement restrictor 76in association with a tool 78. As shown in FIG. 32, the cementrestrictor 76 includes channels through which one or more wires 80 areslidably disposed. The tool 78 includes a slidable portion 82 that canbe moved toward the cement restrictor 76 to cause the wires to slidewithin the channels and extend from a free end of the cement restrictoras shown in FIG. 33. The portion of the wires free of the channels canbe shaped as hooks or barbs 84 for bone engagement. The barbs alsoprevent the wire ends from retracting back into the channels. Followinga temperature change, the cement restrictor 76 assumes an expandedconfiguration as shown in FIG. 34. Subsequently, the tool 78 isseparated (such as by twisting) from the cement restrictor 76 as shownin FIG. 35.

Turning now to FIG. 36, a bistable structure 86 is shown in a firststate. FIG. 37 illustrates the bistable structure 86 in a second state.A deformable wire 88 is embedded within a biocompatible material toprovide stable first and second states. In addition to the heat andstress applications described above, if the wire 88 is a memorymaterial, an electric current can be used to cause a configurationtransition. In one embodiment, a 250 miliamp current is adequate toactivate the wire 88. Although a single bistable structure 86 can act asa cement restrictor, two or more bistable structures can be joined by alink 90, as shown in FIG. 38, to define a cement restrictor.

FIG. 39 illustrates yet another embodiment of a cement restrictor thatincludes two or more flexible, biocompatible elements 92 joined to eachother by one or more links 94. One or more wires 96 made of memorymaterial are engaged with each of the elements 92. As shown in FIG. 39,the wires 96 are curved inward toward the longitudinal axis of thecement restrictor in a first state which causes outer portions of theelements 92 engaged with the wires to be drawn inward as well. As shownin FIG. 40, when the wires 96 are straight in a second state, they nolonger bow inward. As the wires straighten, the elements 92 are pulledoutward with the wire, causing the cement restrictor to have anincreased diameter in the second state.

In addition to configurations that include both stress and temperatureresponsive shape memory components in a single structure describedabove, FIGS. 41-43 depict a cement restrictor that includes temperatureresponsive components that have different response or transitiontemperatures in a single device. Specifically, FIG. 41 depicts a cementrestrictor having a first portion 100 joined to a second portion 102 bya linking element 104, wherein the first portion has fixed dimensions,and the second portion and the linking element have variable dimensions.Although a single second element and linking element are shown,additional elements that are substantially identical to the secondelement can be added to the structure with linking elements.

In the exemplary embodiment, the first portion 100 includes a standardbiocompatible metal such as Cobalt Chromium, the second portion includesUHMWPE having a Nitinol wire 106 embedded therein, and the linkingelement 104 is a Nitinol wire. It should also be noted that in thisembodiment, as well in as the above described embodiments, a “wire” isunderstood to encompass a variety of cross-sectional geometries rangingfrom flat to square to circular or irregular. The wire 106 within thesecond element 102 has a transition temperature of 98.5° F. while thelinking element 104 has a higher transition temperature of about 110° F.The difference in transition temperatures allows the cement restrictorto be reconfigured from a reduced diameter configuration as shown inFIG. 41 to an increased diameter configuration as shown in FIG. 43. Amaximum transition temperature of 120° F. avoids tissue damage in thepatient. As shown, the wire can extend beyond the edges of the secondportion to improve bone engagement.

In use, the cement restrictor is reconfigured as follows. The firstportion 100 is positioned beyond the isthmus with the aid of a tool (notshown) that is engagable within an optional recess 108 in the firstportion. Patient body heat causes the Nitinol in the second portion 102to straighten, thereby increasing the diameter of the second portion asshown in FIG. 42. The UHMWPE composition chosen is resilient enough tobe influenced to change shape and straighten with the embedded Nitinolwire(s), but still has enough rigidity to withstand cementpressurization. A heat gun (not shown) with a long nozzle that blows hotair (greater than 99° F. and less than 120° F.) is placed in the canalof the bone close to the first portion. This heat causes the activationof the linking element to decrease its length and to force thehorizontal, now straightened, second portion closer to the first portionand also further into the narrower section of the isthmus. Analternative method to using a heat gun is to heat the wire with aboutone quarter ampere, since the resistance of the wire is proportional tothe diameter of the wire (i.e. for a 0.006 inch diameter wire, 1ohm/inch can be utilized). High frequency electromagnetic waves orultrasound can also be used to activate the wire(s).

Although the invention has been shown and described with respect toexemplary embodiments thereof, various other changes, omissions andadditions in form and detail thereof may be made without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A cement restrictor comprising: a fluidimpermeable configurable structure having at least one elongatedeformable member disposed therein, the at least one deformable memberbeing made of a shape memory material.
 2. The cement restrictor of claim1, wherein the structure is made of a biocompatible material.
 3. Thecement restrictor of claim 2, wherein the biocompatible material ispolyethylene.
 4. The cement restrictor of claim 2, wherein the elongatedeformable member is a wire.
 5. The cement restrictor of claim 4,wherein the wire is helical.
 6. The cement restrictor of claim 1,wherein the configurable structure includes a cylindrical biocompatiblebody.
 7. The cement restrictor of claim 1, wherein the configurablestructure is bistable.
 8. The cement restrictor of claim 7, wherein theconfigurable structure can be transitioned between a first stable stateand a second stable state by application of an electric current.
 9. Thecement restrictor of claim 1, wherein the configurable structure issubstantially conical and has a length along a longitudinal axis and adiameter extending radially from the longitudinal axis.
 10. The cementrestrictor of claim 9, wherein, the configurable structure includes atleast one channel in which the elongate deformable member is slidablydisposed.
 11. The cement restrictor of claim 10, wherein at least aportion of the elongate deformable member extends from a portion of theconical structure.
 12. The cement restrictor of claim 11, wherein theelongate deformable member includes a barbed periphery.
 13. The cementrestrictor of claim 1, wherein the configurable structure has a firstend along a longitudinal axis and a second end opposite the first end,and wherein heating the configurable structure causes the second end tohave a greater diameter than the first end.
 14. The cement restrictor ofclaim 1, wherein the configurable structure has a first end along alongitudinal axis and a second end opposite the first end, and whereincooling the configurable structure causes the second end to have agreater diameter than the first end.
 15. The cement restrictor of claim1, wherein the configurable structure has a first end along alongitudinal axis and a second end opposite the first end, and whereinstressing the configurable structure causes the second end to have agreater diameter than the first end.
 16. A cement restrictor comprising:a fluid impermeable configurable structure having a substantiallyconical shape and a length along a longitudinal axis and a diameterextending radially from the longitudinal axis, and wherein the fluidimpermeable configurable structure has a plurality of elongatedeformable members disposed therein, and it includes a plurality ofchannels within which one of the plurality elongate members is slidablydisposed.
 17. The cement restrictor of claim 16, wherein at least aportion of each of the elongate deformable members extends from aportion of the conical structure.
 18. The cement restrictor of claim 17,wherein each of the elongate deformable members includes a barbedperiphery.